How to Install Door Casing
Installation is Easy
Get picture perfect miters every time.
Door casing, door trim, door molding, or even door moulding (depending on what part of the world you hail from) are simply the pieces that go around the door jamb to finish off and cover up the gap between the rough opening and the door jamb. They are available in a variety of profiles and sizes including pre-finished and paint grade. We’re going to cover the process I use to get perfect mitered corners, even if the walls you are working with are less than perfect.
Start with The Header
The header is the piece that goes at the top of the door. Cut the length of the short point to short point of the miters to be about 1/2″ longer than the inside measurement of the jamb. That way we will end up with 1/4″ reveal between the inside edge of the jamb and the casing. Cut both miters to 45°. It goes without saying that hands down the best tool for cutting accurate miters is a miter saw.
Apply the Header
No need to measure the reveal, just eyeball both ends and match the reveal at the top. Drive one 18 gauge 1″ brad nail in the center to tack the casing to the jamb. With the casing tacked in the center, it’s easy to make sure the reveal along the top is even before driving a few more nails into the jamb. I usually put 5 along the bottom and then switch to 2″ nails and put 2 or 3 into the framing above the door.
Mark Don't Measure
I suppose the subtitle above might have benefited with the proper use of punctuation, but I like it anyway. And no, there wasn’t a guy named Mark on our crew who didn’t measure. The best and most accurate way to figure out the length of the casing legs is to mark them in place. Set the bottom either on the finished floor (for hardwood or tile), or on a 1/2″ thick piece of scrap for carpet. With the leg overlapping the header, use a utility knife to make a mark on the inside edge of the casing leg.
This is the mark made by the knife that shows us the exact length of the short point of the miter. You can darken it in with a pencil to make it easier to see. Cut the leg a freckle long (leave the mark) with the miter saw set to 45°.
Test the Fit
This is why we left the cut just a little long. If the miter looks perfect, just cut it again with the same 45° setting on the saw right at the mark and we are good to go. If however it’s not perfect, like in this example, we can adjust the cut to make it perfect. You can see that the 45° angles line up (because we did an excellent job hanging the door and it’s square) but we have a small gap on the front because the pieces are touching first along the back. We need to make a compound cut on the leg where it will have a slight bevel on it while keeping the miter angle at 45°.
The Back Cut
No need to overthink the back cut. And you don’t need a dual bevel compound miter saw to pull off this cut. Since we want to cut a little more off the back than the front, we can simply shim the casing up closer to the saw. Here I’m using a carpenter’s pencil to change the bevel angle on the casing. Because of the nature of compound angles, we’ll need to adjust the miter angle slightly to end up with the true 45° angle that we verified works well with the header. To set the angle, keep the casing angled and line up the saw blade with the miter cut. For a small amount of bevel like this, it ends up being about 44 1/2°.
The Back Cut
Make sure to go slow when making the cut. If you power through the cut too quickly, the casing will flex down and you’ll end up with a cut that is anything but straight.
Test the Fit Again
Test fit the new cut to see how we did. It’s almost perfect now. We can make this work with another little trick when we apply it.
Before installing the leg casing, apply some yellow glue to the joint because that will keep the miter from even opening up down the road.
Apply the Casing
Install the leg the same way as the header, starting at the top and working your way down. Eyeball the reveal as you go to keep it even along the entire length. Sometimes a super thin shim is needed to line up the miters perfectly.
Pin It Together
Pin the miter together. A 23 gauge micro pinner is the perfect tool for the job. Its tiny 23 gauge nail is so small that it will never split the wood, but it has enough holding power to keep the joint intact until the glue dries.
Leave the shim in until the glue has dried.Then you can pull it out and the miter joint will look this good for a long time.
It’s ready for caulking, spackle and paint.
A Note About Safety
Safety is important. I can’t say it any better than my all-time favorite woodworker/ TV host, Norm Abram, so I’ll just leave you with his famous quote:
“Be sure to read, understand, and follow all the safety rules that come with your power tools. Knowing how to use your power tools properly will greatly reduce the risk of personal injury. And remember this: there is no more important safety rule than to wear these (Norm points to his glasses) — safety glasses.”
-Norm Abram, New Yankee Workshop